Corona shielding means for magnetic cores



y 1967 H. G. FISCHER ETAL 3,333,220

CORONA SHIELDING MEANS FOR MAGNETIC CORES Filed Jan. 28, 1966 2 Sheets-Sheet l FIG.l.

INVENTORS Heinz G. Fischer and Clifford J. Bell ATTORNE July 25,-1967 H.G. FISCHER ETAL 3,333,220

CORONA SHIELDING MEANS FOR MAGNETIC CORES Filed Jan. 28, 1966 v v 2 Sheets-Sheet 2 United States Patent 3,333,220 CORONA SHIELDING MEANS FOR MAGNETIC CORES Heinz G. Fischer, Mnncie, and Clifford J. Bell, Mount Pleasant Township, Yorktown, Ind., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 28, 1966, Ser. No. 523,679 Claims. (Cl. 336-84) This invention relates in general to corona shielding means for magnetic cores, and more specifically to corona shielding means for shielding certain edges of magnetic cores used in electrical inductive apparatus.

Corona discharge or streamers in electrical inductive apparatus is undesirable due to its degrading effect upon the surrounding electrical insulation. Certain edges of the magnetic core structures used in electrical inductive apparatus, such as transformers, present sharp electrodes facing the electrical windings, which may initiate corona discharges if not properly shielded.

Prior art attempts to shield these edges of the magnetic core have either been very expensive, ineffective, or both, due to the difficulty in properly locating the shielding means relative to the magnetic core edge to be shielded, and due to the difficulty of maintaining the shielding means in the desired location once placed in position, during the subsequent manufacturing stages and assembly of the electrical apparatus. Welding the shielding means to the T-beam or other suitable structure assures a fixed location, but is costly and requires diligent polishing of the welding seams to prevent corona discharges from occurring at sharp points on the weld seam.

Therefore, it would be desirable to provide new and improved means for shielding certain edges of the magnetic core of electrical inductive apparatus which is inexpensive, easy to position during assembly, secure against movement during subsequent assembly steps in the manufacture of the apparatus, and of one-piece construction which eliminates any requirement of welding to complete the shield and holdit in the desired position.

Accordingly, it is an object of the invention to provide a new and improved corona shield for certain edges of the magnetic core of electrical inductive apparatus.

Another object of the invention is to provide a new and improved corona shield for certain edges of a magnetic core, which shield is of a one-piece construction.

A further object of the invention is to provide a new and improved corona shield for certain edges of a magnetic core which facilitates the proper location of the shield relative to the edge to be shielded.

Still another object of the invention is to provide a new and improved corona shield for certain edges of a magnetic core of electrical inductive apparatus, which facilitates accurate and positive location of the shield, and which is not subject to movement during subsequent assembly steps in the manufacture of the apparatus.

Briefly, the present invention accomplishes the above cited objects by forming the corona shielding means, for each magnetic core edge to be shielded, of a thin continuous metallic sheet, which has a length dimension substantially equal to the length of the edge to be shielded, and predetermined Width and thickness dimensions. The sheet is folded along its length dimension to form a portion intermediate its width dimension which has a smooth curved surface. The smooth curved portion is 3,333,226 Patented July 25, 1967 ice be slightly less than ninety degrees (90), with approximately eighty degrees (80) being suitable. The corona shielding means is disposed to shield an edge of a magnetic core for electrical inductive apparatus, which includes a plurality of stacks of metallic laminations and electrical windings which extend through the winding opening in the magnetic core.

In one embodiment of the invention, the first leg of the shielding means is disposed to enter the winding opening adjacent the edge of the magnetic core it is to shield, and is held firmly in place between the electrical windings and magnetic core. The second leg of the shielding means is disposed against the outer lamination of its associated stack of laminations, forming an angle of substantially ninety-degrees (90) with the first leg portion once in position in the core-winding assembly. The set in the shielding means which causes the angle between the first and second leg portions to be less than ninety degrees (90) when unrestrained, assures a tight fit for the first and second leg portions against the adjacent surfaces of the magnetic core. When the leg portions of the shielding means are locked in position, the curved portion of the shielding means will automatically be disposed in spaced relation with the core edge it is to shield, and it will not be subject to movement during subsequent assembly operations performed on the apparatus.

Further objects and advantages of the invention will become apparent from the following detailed description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a perspective view of a corona shield constructed according to the teachings of the invention;

FIG. 2 is a perspective view of electrical inductive apparatus utilizing the corona shields shown in FIG. 1;

FIG. 3 is an elevational view, in section, illustrating how the corona shields are placed during the assembly of the magnetic core shown in FIG. 2;

, FIG. 4 is a elevational view, in section, illustrating the corona shields in place relative to a completed section of the magnetic core shown in FIG. 2;

FIG. 5 is an elevational view of electrical inductive apparatus illustrating the use of the corona shield of FIG. 1 in another embodiment of the invention;

FIG. 6 is a sectional view of the magnetic core and corona shielding arrangement shown in FIG. 5, taken along the line VI-VI; and

FIG. 7 is a sectional view of a magnetic core and corona shielding arrangement illustrating another embodiment of the invention.

Electrical inductive apparatus, such as transformers, include electrical windings disposed in inductive relation with a magnetic core. The magnetic core includes at least one opening for receiving electrical windings, and is formed of a plurality of stacks of metallic laminations, such as grain oriented silicon steel, with the stacks being arranged to form a complete magnetic circuit. The edges of the magnetic core which face the windings form sharp electrodes which are a possible source of corona discharge and streamers. Corona may seriously erode and degrade' the solid insulation on the adjacent windings, as well as cause breakdown of the insulating oil, causing bubbles of gas in the oil where corona can form even more easily, accelerating the breakdown process.

In order to prevent corona formation on these edges of the magnetic core, the edges should be Shielded with a smooth, rounded, equipotential metallic shielding means. The shielding means should be relatively inexpensive, easily placed during the assembly of the magnetic core, and firmly secured in the desired location, preventing movement of the shielding means during subsequent steps in the manufacture and assembly of the electrical inductive apparatus. Further, the shielding means should preferably be of one piece construction and should be secured in the desired position without necessity of welding or other auxiliary fastening means.

FIGURE 1 illustrates new and improved corona shielding means 10 constructed according to the teachings of the invention, for shielding an edge of a magnetic core. Corona shielding means 10 is of one piece construction, and is formed of a metallic sheet 12 having a length dimension L, which is substantially equal to the length of the magnetic core edge to be shielded, and predetermined width and thickness dimensions. Corona shielding means 10 is preferably formed of a non-magnetic metal having a resistivity which is low enough to quickly distribute electrical charging currents, but high enough to limit eddy currents caused by the magnetic stray field. A non-magnetic stainless steel, such as type 304, has been found to be excellent. The thickness dimension T of the corona shielding means should be as thin as possible, to keep its electrical resistance relatively high and thus prevent excessive heating due to the magnetic stray field of the transformer, and still provide a structural shape which will strongly resist change in its dimensions once formed to the desired shape. Non-magnetic stainless steel & to A of an inch thick has been found to provide the necessary structural strength, and still minimize heating of the shield due to magnetic stray fields.

The metallic sheet 12 is folded along its length dimension L, to form a curved portion 14 intermediate the width dimension of the sheet 12, forming a structure having an overall width dimension W. The width dimension W, in general, is determined by the width of the metallic laminations which make up the stack whose edge is to be shielded, and in certain embodiments of the invention may be substantially equal to the lamination width if desired.

The curved portion 14 may be formed to have a constant radius R, for example three-fourths (%i) of an inch has been found suitable on certain applications, but this dimension is not critical. In fact, the curved portion 14 may be complex, having a continuously varying radius if desired.

The curved portion 14 is joined on one side by a first leg portion 16, and on the other side by a sec-nd leg portion 18. The first leg portion 16 joins the curved portion 14 smoothly, in a substantially tangential manner. If the dotted line 22 is considered to be a true tangent to the curved portion 14, the first leg portion 16 should preferably be joined to the curved portion 14 to form an angle 24 of a few degrees with dotted line 22, for purposes which will be hereinafter explained. The size of the angle 24 is not critical. Ten degrees (10) has been found to be suitable, but it may vary quite widely on either side of this value.

The sec-0nd leg portion 18 joins the curved portion 14 at a predetermined angle 20. The size of angle 20 is that angle necessary to form an angle 26 of substantially ninety degrees'(90) between the second leg portion 18 and the dotted line 22. If the curved portion 14 has a constant radius R and is exactly a half circle, the angle 20 would be substantially ninety degrees (90). The angle 28 between the first and second leg portions 16 and 18, respectively, is less than ninety degrees (90).

The length of the first leg portion 16 is not critical, with 3 to inches being suitable in most applications.

FIGURE 2 is a perspective view of a transformer 30, shown partially cut away, illustrating the use of the corona shield of FIGURE 1 in a three-phase electrical transformer of the shell-form type. Transformer 30 includes a core-winding assembly 31 disposed in a suitable casing or tank 33. Tank 33 may contain a suitable fluid dielectric, if desired. More specifically, transformer 30 includes first and second magnetic core sections 32 and 34, respectively, disposed in side-by-side relation. Each magnetic core section includes a plurality of stacks 36 of superposed metallic laminations 38. The stacks 36 are disposed to form complete magnetic circuits about a plurality of openings for receiving electrical windings. Since transformer 30 is a three-phase transformer of the shell-form type, each magnetic core section 32 and 34 will have three winding openings, such as opening 39. The Winding openings are perpendicular to the plane of the superposed laminations 38.

Transformer 30 includes three electrical winding phases 40, 42 and 44, which include a plurality of pancake or disc type coils 46 which are stacked in side-by-side relation with the openings in the coils 46 in alignment, forming opening 47 for receiving the magnetic core sections 32 and 34. The magnetic core sections 32 and 34 are built up around the electrical winding phases 40, 42 and 44, with each magnetic core section entering the opening 47 in the electrical windings, and the windings entering the winding openings, such as opening 39, in the magnetic core sections 32 and 34.

In order to shield the edges of the magnetic core sections 32 and 34, at each end of the winding openings, corona shielding members 10, of the type shown in FIG- URE 1, are disposed to enclose certain of the edges. As shown in FIGURE 2, all of the edges of the upper end of the openings are enclosed, except the edges facing the adjacent magnetic core section. The same is true for the bottom end of the openings. The edges facing the adjacent magnetic core section could be enclosed with corona shielding members 10, if desired, but it is not essential if the conventional T-beam and/or wedges of vertically stacked metallic laminations are inserted in the opening 47 in the electrical windings between the magnetic core sections 32 and 34 and the winding, and at the corresponding location at the bottom of the core sections 32 and 34. The T-beam, such as T-beam 50 shown at the bottom opening, and wedges of metallic laminations adequately shield the inner edges of the openings, making it unnecessary to provide auxiliary shielding. If, however, a dilferent method of supporting the windings is used which would not provide adequate shielding for this inner edge of the core opening, the corona shielding members 10 could be used to shield all of the edges of the openings in the magnetic core.

The corona shielding members 10 are easily placed in position while the transformer 30 is being assembled, and they will retain their position during subsequent steps in the assembly of the transformer.

To illustrate how the corona shilelding members 10 are placed in position to surround the core edge it is to shield, with a smooth, rounded equipotential surface which distributes electrical stresses and prevents. them from concentrating on the sharp core edge, a cross-section of one of the stacks of laminations which make up core sections 32 and 34 is shown in FIGURE 3.

In the assembly of the magnetic core sections, such as magnetic core section 34, around the electrical windings, such as electrical winding 40, a member 52 formed of wood or other suitable electrical insulating material, is disposed on the core supporting means 55. The member 52 has a cut-out portion at one end thereof for receiving the curved portion 14 of the corona shielding member 10.

The corona shielding member 10 is disposed with its second leg portion 18 on member 52, its curved portion 14 extending into the cut-out portion of member 52, and its first leg portion 16 extending vertically upward along the electrical winding 40. The magnetic core stack 36 is then built up, starting on the corona shielding member 10. Insulating means 54 is disposed between the leg portion 16 of corona shielding member 10 and the ends of v the individual laminations 38. Insulating means 54 may be an epoxy coating on the leg portion 16, or any other form of electrical insulation which will prevent leg portion 16 from shorting the ends of the laminations 38. As the laminations 38 are placed in position, leg portion 16 will be moved to a substantially vertical position from its slight inward bend, hereinbefore described, which takes care of tolerances in fabrication of the punchings or laminations 38, and ensures a tight fit of the leg portions 16 and 18 against the outer surfaces of the magnetic core structure 36. 7

When the stack 36 reaches a point a few inches from its finished height, the laminations 38 are stacked to provide a larger opening between the ends of the laminations 38 and the winding 40, to allow room for the first leg portion 16 of corona shielding means to be inserted. Once the stack 36 is completed to the desired height, corona shielding member 10' is disposed with its second leg portion 18' on the top lamination 38, and its first leg portion 16' entering the space provided between winding 40 and the ends of the laminations 38. Insulating means 56 is disposed to electrically insulate the ends of the laminations 38 from leg portion 16'. The laminations 38 at the top portion of the stack 36, which were purposely off-set from the lower portion of the stack 36, are now moved into alignment with the other laminations, which moves the first leg portion 16' to a substantially vertical position, firmly securing the corona shielding means 10' between the ends of the laminations 38 and the electrical winding 40.

FIGURE 4 illustrates a cross-sectional view of stack 36, similar to the view shown in FIGURE 3, except FIG- URE 4 shows the final assembly view of the stack, after the top laminations 38 have been placed in final position. An insulating member 60, similar to member 52, is placed over the leg portion 18, and means 62 is placed over member 60 to securely hold the stack 36, along with corona shielding members 10 and 10, in the desired assembled relation.

While the corona shielding means has been shown and described relative to a three-phase transformer of the shell-form type, the invention is equally applicable to single-phase transformers of the shell-form type, and to single and polyphase transformers-of the core-form type.

For example, FIG. 5 illustrates an elevational view of a three-phase transformer 70 of the core-form type, which utilizes the corona shields 10 shown in FIG. 1.

Transformer 70 includes a core-winding assembly 72, having a magnetic core 74 formed of a plurality of stacks of superposed metallic laminations 76. The plu- 'rality of stacks of laminations are arranged about winding openings or windows 78 and 80, forming spaced, parallel winding leg members 82, 84 and 86, whose ends are connected by yoke members 88 and 90.

Core-winding assembly 72 also includes three electrical winding phases 92, 94 and 96, which include high and low voltage windings concentrically disposed about Winding leg members 82, 84, and 86, respectively.

In transformers of the core-form type, at least the edges of the Winding openings 78 and 80, formed by the leg members 82, 84 and 86 must be shielded, as well as the remaining edges of the leg members. Accordingly, corona shields 10 are disposed about each edge of each of the leg members 82, 84, and 86.

If the leg members 82, 84 and 86 are of rectangular cross-section, a cross-sectional view of leg member 82, taken along lines VIVI would appear as shown in FIG. 6, with the first leg portion of the corona shield 10 being disposed along the edges of the laminations 76, and insulated therefrom by insulating means 98, and the second leg portion being disposed along the plane of the outside laminations in the stack. 5

If the leg members 82, 84 and 86 were to be formed in a cruciform cross-section, the corona shields 10 would be disposed about each of the edges of the leg member, such as shown about the eight edges of a leg member 180 shown in cross-section in FIG. 7.

In most core-form transformers the core end frames (not shown) are made of metal, and provide adequate shielding for the edges of the core window openings 78 and 80 formed by the edges of the yoke members 88 and 90. However, in the event the core end frames are made of an electrical insulating material, these edges of the core windows may also be shielded by corona shielding members 10, as shown in FIG. 5.

In summary, there has been disclosed new and improved corona shielding means for shielding certain of the edges of magnetic cores used with electrical inductive apparatus. The corona shielding means is of one piece construction, is easily placed in the proper position relative to the edge to be shielded, and is firmly held in the desire-d position without any possibility of movement during subsequent steps in the assembly of the apparatus. All of these advantages are obtained with a low cost, sheet metal member which does not require welding to assure that it will remain in the desired position.

Since numerous changes may be made in the above-described apparatus and different embodiments of the invention may be made without departing from the spirit there of, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative, and not in a limiting sense.

We claim as our invention:

1. Electrical inductive apparatus comprising magnetic core means; said magnetic core means including a plurality of stacks of superposed metallic laminations arranged to form a magnetic circuit about at least one opening for receiving electrical windings; said at least one opening being defined at each end of the opening by certain edges of the plurality of stacks of metallic laminations; electrical windings disposed in inductive relation with said magnetic core means, extending through said at least one opening in said magnetic core means; corona shielding means disposed to enclose certain of the edges of one said magnetic core means; said corona shielding means for each of the certain edges including a metallic sheet member having a length substantially equal to the length of the edge to be shielded; said metallic sheet member being folded along its length, forming a curved portion; the curved portion of said metallic sheet member being terminated on each side by first and second leg portions; the first leg portion of said metallic sheet member being substantially tangential to the curved portion, and the second leg portion forming that angle with the curved portion which provides an angle of substantially 90 between said first and second leg portions; each of said metallic sheet members being disposed with its first and second leg portions on opposite sides of the edge of the stack of metallic laminations it is to shield, and the curved portion of each of said metallic sheet members enclosing the edge to be shielded.

2. The electrical inductive apparatus of claim 1 wherein said corona shielding means is given a predetermined set which would cause the angle between the first and second leg portions to be less than 90 if unrestrained.

3. The electrical inductive apparatus of claim 1, including insulating means disposed between the leg of said corona shielding means which is adjacent the ends of the superposed metallic laminations, preventing the corona shielding means from shorting the stacked metallic laminations.

4. The electrical inductive apparatus of claim 1, wherein said magnetic core means is shell-type construction having two spaced magnetic core sections, each having at least one opening for receiving said electrical windings, said corona shielding means being disposed adjacent certain of the edges which define the ends of the openings in said magnetic core means, with the first leg portions entering the winding openings and the second leg portion disposed adjacent to and parallel with the outer laminations of the stacks of metallic laminations.

5. The electrical inductive apparatus of claim 4, wherein the first leg portion of said metallic sheet members is secured in position by said electrical windings and said magnetic core means.

6. The electrical inductive apparatus of claim 4, wherein the first leg portion of each of said metallic sheet memhers is secured in position by said electrical windings and said magnetic core means, and including means disposed on the second leg portion 'which mechanically protects the curved portion of said metallic sheet member.

7. The electrical inductive apparatus of claim 1 wherein said magnetic core means is of single-phase shell-type construction having two spaced magnetic core sections, each having an opening for receiving said electrical windings, said corona shielding means being disposed adjacent certain of the edges which define the two openings in said magnetic core means, with the first leg portions entering the openings and the second leg portions being disposed adjacent to and parallel with the outer laminations of the stacks of metallic laminations.

8. The electrical-inductive apparatus of claim 1, wherein said magnetic core means is of three-phase shell-type construction having two spaced magnetic core sections, each having a plurality of openings for receiving said electrical windings, said corona shielding means being disposed adjacent certain of the edges which define the ends of the plurality of openings in said magnetic core means.

9. The electrical inductive apparatus of claim 1', wherein said magnetic core means is of single-phase core-type construction having yoke and winding leg members, said corona shielding means benig disposed to enclose at least the edges of said Winding leg members.

.10. The electrical inductive apparatus of claim 1, wherein said magnetic core means is of three-phase coretype construction having yoke and winding leg members, said corona shielding means benig disposed to enclose at least the edges of said winding leg members.

References Cited FOREIGN PATENTS 176,311 1922 Great Britain.

LEWIS H. MYERS, Primary Examiner.

20 T. J. KOZMA, Assistant Examiner. 

1. ELECTRICAL INDUCTIVE APPARATUS COMPRISING MAGNETIC CORE MEANS; SAID MAGNETIC CORE MEANS INCLUDING A PLURALITY OF STACKS OF SUPERPOSED METALLIC LAMINATIONS ARRANGED TO FORM A MAGNETIC CIRCUIT ABOUT AT LEAST ONE OPENING FOR RECEIVING ELECTRICAL WINDINGS; SAID AT LEAST ONE OPENING BEING DEFINED AT EACH END OF THE OPENING BY CERTAIN EDGES OF THE PLURALITY OF STACKS OF METALLIC LAMINATIONS; ELECTRICAL WINDINGS DISPOSED IN INDUCTIVE RELATION WITH SAID MAGNETIC CORE MEANS, EXTENDING THROUGH SAID AT LEAST ONE OPENING IN SAID MAGNETIC CORE MEANS; CORONA SHIELDING MEANS DISPOSED TO ENCLOSE CERTAIN OF THE EDGES OF ONE SAID MAGNETIC CORE MEANS; SAID CORONA SHIELDING MEANS FOR EACH OF THE CERTAIN EDGES INCLUDING A METALLIC SHEET MEMBER HAVING A LENGTH SUBSTANTIALLY EQUAL TO THE LENGTH OF THE EDGE TO BE SHIELDED; SAID METALLIC SHEET MEMBER BEING FOLDED ALONG ITS LENGTH, FORMING A CURVED PORTION; THE CURVED PORTION OF SAID METALLIC SHEET MEMBER BEING TERMINATED ON EACH SIDE BY FIRST AND SECOND LEG PORTIONS; THE FIRST LEG PORTION OF SAID METALLIC SHEET MEMBER BEING SUBSTANTIALLY TANGENTIAL TO THE CURVED PORTION, AND THE SECOND LEG PORTION FORMING THAT ANGLE WITH THE CURVED PORTION WHICH PROVIDES AN ANGLE OF SUBSTANTIALLY 90* BETWEEN SAID FIRST AND SECOND LEG PORTIONS; EACH OF SAID METALLIC SHEETS MEMBERS BEING DISPOSED WITH ITS FIRST AND SECOND LEG PORTIONS ON OPPOSITE SIDES OF THE EDGE OF THE STACK OF METALLIC LAMINATIONS IT IS TO SHIELD, AND THE CURVED PORTION OF EACH OF SAID METALLIC SHEET MEMBERS ENCLOSING THE EDGE TO BE SHIELDED. 